A long term evolution (LTE) system which is a 3.9G wireless communication system of a cell-phone has been almost standardized, and an LTE-Advanced (LTE-A, which is also called an IMT-A) which is a 4G wireless communication system more advanced than the LTE system recently started to standardize by the 3rd Generation Partnership Project (3GPP). For an uplink (communication from a mobile station to a base station) of the LTE-A system, since the system has been further extended from the LTE system, a transmission diversity scheme using a plurality of transmitting antennas which has not been defined in an uplink of the LTE is scheduled to be defined in view of power consumption of the mobile station or the cost.
Meanwhile, a downlink (communication from the base station to the mobile station) of the LTE employs an orthogonal frequency division multiplexing (OFDM) scheme in which each subcarrier is independently modulated. Further, since the base station transmits a signal and thus a restriction to power consumption is weak, the transmission diversity scheme has been already defined. For example, when the number of transmitting antennas is two, a space frequency block code (SFBC) and a cyclic delay diversity (CDD) have been defined (for example, Non-Patent Document 1).
FIG. 7 is a diagram illustrating a concept of the SFBC. Referring to FIG. 7, transmission data is subjected to SFBC coding by an SFBC coding section 1000 and transmitted from a transmitting antenna 1001 and a transmitting antenna 1002. In SFBC coding, for example, when two antennas are used, a subcarrier S1003 has a relation in which a minus is put to a complex conjugate of a subcarrier S1000, and a subcarrier S1001 and a subcarrier S1002 have a complex conjugate relation. That is, if the amplitude of a (2k−1)-th subcarrier input to original transmission data (the SFBC coding section 1000) is S(2k−1), and the amplitude of a 2k-th subcarrier is S(2k), the amplitudes of the subcarrier S1000 to the subcarrier S1003 are represented by the following equations.
[Equation 1]S1(2k−1)=S(2k−1)  (1)
[Equation 2]S1(2k)=S(2k)  (2)
[Equation 3]S2(2k−1)=−S*(2k)  (3)
[Equation 4]S2(2k)=S*(2k−1)  (4)
Here, S1(2k−1) and S1(2k) are the amplitudes represented by complex numbers of (2k−1)-th and 2k-th subcarriers transmitted from the transmitting antenna 1001, and S2(2k−1) and S2(2k) are the amplitudes of (2k−1)-th and 2k-th subcarriers transmitted from the transmitting antenna 1002.
The transmission signal transmitted as described above is received by a receiving antenna 1003, and SFBC-decoded data is extracted by an SFBC decoding section 1004. If complex gains of a channel of the transmitting antenna 1001 to the receiving antenna 1003 in the (2k−1)-th and 2k-th subcarriers are H1(2k−1) and H1(2k), and complex gains of a channel of the transmitting antenna 1002 to the receiving antenna 1003 in the (2k−1)-th and 2k-th subcarriers are H2(2k−1) and H2(2k), receiving signals R(2k−1) and R(2k) are represented by the following equations.
[Equation 5]R(2k−1)=H1(2k−1)S(2k−1)+H2(2k−1)S*(2k)  (5)
[Equation 6]R(2k)=H1(2k)S(2k)−H2(2k)S*(2k−1)  (6)
When S(2k−1) and S(2k) are expressed by the receiving signals in view of Equations 5 and 6, the following equation is derived. Actually, noise from a receiving device or interference from a neighboring cell is included, but a description thereof is herein omitted for simplifying a description.
[Equation 7]H*1(2k−1)R(2k−1)−H2(2k)R*(2k)=(|H1(2k−1)|2+|H2(2k)|2)S(2k−1) +(H*1(2k−1)H2(2k−1)−H*1(2k)H2(2k))S*(2k)  (7)
[Equation 8]H2(2k−1)R*(2k−1)+H*1(2k)R(2k)=(|H2(2k−1)|2+|H1(2k)|2)S(2k) +(H*1(2k−1)H2(2k−1)−H*1*(2k)H2(2k))S*(2k−1)  (8)
In Equations 7 and 8, when the subcarrier “2k−1” is almost the same in channel gain as the subcarrier 2k (a variation is sluggish), it can be regarded that H1(2k−1) is equal to H1(2k), and H2(2k−1) is equal to H2 (2k). Thus, in this case, the second terms of the right sides of Equations 7 and 8 become zero, and thus it is possible to obtain an effect of maximum ratio combining (MRC) in which the magnitudes of channel gains from the transmitting antennas are weighted and combined.
FIG. 8 is a diagram illustrating an example of the CDD. In the case of the CDD shown in FIG. 8, when two transmitting antennas are used, only one antenna includes a cyclic shift section 1005. Further, a transmitting signal 1009 from a transmitting antenna 1007 is cyclic-shifted by the cyclic shifting section 1005 compared to a transmitting signal 1008 transmitted from a transmitting antenna 1006 and shifted by two symbols compared to the transmitting signal 1008. At a receiving side, signals from all transmitting antennas are added and received by a receiving antenna 1010. Since the receiving side regards that the transmitting signal from the transmitting antenna 107 has been shifted by the number of symbols cyclic-shifted by the channel, a maximum delay time of an impulse response of the channel equivalently increases. For this reason, by increasing a frequency variation, a frequency having a good channel gain is mixed with a frequency having a bad channel gain. Thus, the entire signal band is prevented from dipping into a frequency band having a bad channel gain. Performance of the SFBC level is not shown, but it is effective at the time of moving at a high speed or when a variation in the channel cannot be expected at all.